Loss of endothelial barrier function is an important characteristic of Acute Lung Injury (ALl). The transcellular transport of albumin and other macromolecules via endothelial caveolae is a factor contributing to endothelial barrier function. We have identified specific interactions between caveolin-1 (a caveolar protein), the heterotrimeric G protein Gi, and Src kinase in the mechanism of caveolae-mediated endocytosis. The goals of Project 4 are to define the role of caveolin-1 as an organizer and regulator of signal transduction cascades essential for plasmatemmal vesicle trafficking and the protein-protein interactions that regulate albumin permeability via transcytosis. The studies in Project 4 will address the following specific aims. Specific Aim #1: To determine the role of the heterotrimeric G protein, Gi, in signaling ceaveolae-mediated endocytosis and transendothelial albumin permeability in endothelial monolayers; Specific Aim #2: To determine the role of Src activation of the GTPase, dynamin-2, in signaling caveolae-mediated endocytosis and transendothelial albumin permeability; Specific Aim #3:To address the component of thrombin/Protease Activated Receptor-1-induced increase in lung vascular permeability resulting from internalization of caveolae and transcelinlar albumin transport. Thus, Project 4 will identify the receptor-coupled signals activating Src, the phosphorylation targets of Src signaling caveolae fission (specifically, caveolin-1 and dynamin-2), and the role of Src activation in regulating transcellular permeability. To address the in vivo relevance and functional significance of these studies in pulmonary microvascular endothelial cells, experiments will also be made, wherever possible, in intact mouse lung models. Studies will employ approaches in both imaging (i.e., using fluorescent probes and electron microscopic assessment) and physiology (i.e., determination of endothelial permeability in monoayers and mouse lung models) to address the role of caveolae-mediated endocytosis in activating increased albumin permeability. Thus, these studies will elucidate the signaling mechanisms that regulate caveolae internalization and plasmalemmal vesicle trafficking, and thus contribute to the mechanism of transendothelial albumin permeability in lungs. The achievement of these objectives will lead to tthe elucidation of the signals regulating caveolae-mediated endocytosis and its role in contributing to the thrombin-induced increase in lung vascular permeability. With the identification of novel signaling pathways, it may be possible to develop therapeutic strategies that specifically target signals leading to inappropriate increase in lung vascular permeability.